As a hydrogen absorbing alloy to be used for a secondary battery, “rare earth-Ni-based hydrogen absorbing alloys” containing a CaCu5 type crystal as a main phase and “Laves phase-based hydrogen absorbing alloys” containing Ti, Zr, V, and Ni have been known. As a new hydrogen absorbing alloy replacing these hydrogen absorbing alloys, in recent years, “rare earth-Mg—Ni-based alloys” have been drawing attention. It is because that this hydrogen absorbing alloy is reported to have a discharge capacity exceeding that of an AB5 type alloy.
Patent Document 1 discloses an electrode using an LaCaMgNi9 alloy having a PuNi3 type crystal structure.
Patent Documents 2 to 4 disclose that electrodes using rare earth-Mg—Ni-based alloys having a Ce2Ni7 type crystal structure or the like show good hydrogen releasing characteristics while keeping a high hydrogen storage capacity.
For instance, in paragraph numbers 0252 to 0254 of Patent Document 2, a fifteenth hydrogen absorbing alloy is disclosed and the fifteenth hydrogen absorbing alloy is defined by R11-a-bMgaT2b(Ni1-xM7x)z and contains an alloy having at least one phase selected from phases having a Ce2Ni7 type, a CeNi3 type, a Gd2Co7 type, or a PuNi3 type crystal structure and analogous crystal structures as a main phase.
Herein, R1 is at least one element selected from rare earth elements including yttrium; T2 is at least one element selected from Ca, Ti, and Zr; M7 is at least one element selected from Co, Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P, and B; and a, b, x, and z satisfy 0<a≦0.6, 0≦b≦0.5, 0≦x≦0.9, and 2.5≦z≦4.5, respectively. Further, it is described that the “main phase” means that at least one phase selected from phases having the Ce2Ni7 type, CeNi3 type, Gd2Co7 type, or PuNi3 type crystal structure and analogous crystal structures occupies the maximum volume in the hydrogen absorbing alloy or the maximum area in a cross section of the hydrogen absorbing alloy.
Further, the following is disclosed, that is, @) particularly, at least one phase selected from phases having the above-mentioned crystal structures is preferable to exist in 50% or more by the area ratio in the hydrogen absorbing alloy: (ii) if the existence ratio of the phase is less than 50%, it is probable that the hydrogen storage capacity is decreased: (iii) therefore, it is probable that the discharge capacity of a secondary battery having a negative electrode containing the hydrogen absorbing alloy is lowered or the charging and discharging cycle life is shortened: and (iv) the area ratio of the phase is more preferably 60% or higher.
In paragraph numbers 0343 to 0344 of Patent Document 2, it is disclosed that since the fifteenth hydrogen absorbing alloy of Patent Document 2 is defined by a general formula R11-a-bMgaT2b(Ni1-xM7x)z and contains an alloy of at least one phase selected from phases having the Ce2Ni7 type, CeNi3 type, Gd2Co7 type, or PuNi3 type crystal structures and analogous crystal structures as a main phase, the hydrogen absorption and release characteristics such as a hydrogen absorption and release speed are remarkably improved. Further, it is also disclosed that since the secondary battery provided with a negative electrode containing this fifteenth hydrogen absorbing alloy is excellent in the hydrogen absorption and release characteristics of the hydrogen absorbing alloy, the secondary battery has a high capacity and an excellent charging and discharging cycle characteristic.
Patent Document 5 discloses that the cycle characteristics are improved by replacing a part of Ni in a rare earth-Mg—Ni-based hydrogen absorbing alloy with Al.    Patent Document 1: Japanese Patent No. 3015885    Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 11-323469    Patent Document 3: JP-A No. 2001-316744    Patent Document 4: Japanese Patent No. 3247933    Patent Document 5: JP-A No. 2004-221057